1. Field of the Invention
The present invention relates generally to processes for both producing and recovering cyclic esters of hydroxy organic acids, such as lactic acid, glycolic acid, and lactones. More particularly, it concerns methods that use azeotropic distillation for production and recovery of such cyclic esters. It also relates to methods of forming lactide directly from crude fermentation broth containing significant water without a prior dehydration step.
2. Description of Related Art
Cyclic esters of hydroxy organic acids have a number of commercial uses. Cyclic esters such as gamma-butyrolactone are widely used as a feedstock for manufacture of various polymers. Lactide is an important compound as an intermediate for the production of polylactic acid (PLA) plastics which can be biodegraded. Similarly, glycolide can be used in the production of biodegradable polyglycolide plastics (PGA plastics).
Although PLA and PGA polymers have been known for more than half a century, they have recently been attracting much attention, primarily because of their biodegradability. PLA plastics are, for example, being used in packaging, paper coatings, absorbable surgical sutures, vessel implants, and orthopedic pins, among others. PLA and PGA plastics are attractive not only because they biodegrade into carbon dioxide and water, but also because they can be derived from renewable agricultural resources, such as corn or sugar beets. Such polymers can, in certain cases, also be produced using solvent-free processes, which can be more environmentally friendly than certain solvent-based processes used in producing plastics from petrochemicals. In addition, advances in technology are now making it possible to produce these plastics more cheaply, so that they can be produced at a price that is more competitive with plastics produced from petrochemicals. Thus, new and improved sources for biodegradable plastics are highly desirable.
High molecular weight polylactic acid (PLA plastic) can be obtained by the ring-opening polymerization of lactide. Both the chemical purity and the optical purity of lactide (there are different optical isomers of lactic acid and consequently different lactide isomers) used to produce polylactic acid can dramatically affect the nature of the polylactic acid produced from the lactide. Chemical impurities can also diminish the usefulness of lactide when it is a food additive, because impurities can, in certain instances, cause off tastes or cause the acidity of a preserved food to be modified in undesirable ways.
Polyglycolide plastics are produced by a ring-opening polymerization of glycolide, similar to production of polylactic acid polymers from lactide. The properties of the PGA plastics produced from glycolide are dependent on the quality and purity of the substrate.
Lactide is typically produced from lactic acid, and glycolide from glycolic acid. Lactide can be formed from two molecules of lactic acid, with the loss of two molecules of water. Lactide is typically formed either by “ring closing” or from a larger oligomer. In ring closing, a lactic acid dimer eliminates water to form lactide, which is a cyclic diester. In this case, the mechanism is an esterification reaction. Alternatively, the lactide can be formed from a longer oligomer of lactic acid containing, for example, 10 to 20 lactic acid molecules linked together by ester linkages as a short polymer. Elimination of two lactic acid moieties from this chain gives lactide. Such a reaction is a transesterification reaction. Alternatively, lactide can be produced by first forming an ester of lactoyl lactic acid linear dimer with some other species, such as an ethylene glycol-ether. Lactide is then formed by a transesterification reaction with elimination of the lactic acid dimer unit.
Although lactic and glycolic acids can be prepared by chemical synthesis, it is generally more cost efficient to produce such organic acids through fermentation of sugars, starch, or cheese whey, using microorganisms such as Lactobacillus delbrueckii. Such microorganisms convert monosaccharides such as glucose, fructose, or galactose, or disaccharides such as sucrose, maltose, or lactose, into such organic acids. The broth that results from fermentation contains unfermented sugars, carbohydrates, amino acids, proteins, and salts, as well as the desired acids. Some of these materials can interfere with downstream processing of an organic acid, for example processing of lactic acid to form lactide. The organic acid usually therefore must first be recovered from the fermentation broth and in some cases must undergo further purification before it can be used to produce cyclic esters.
Lactic acid or other hydroxyacids or diacids can be converted to polyesters. These polyesters can be recycled via digestion using pressurized water, acid, base, or a combination of such treatments. The products of such digestion can be a mixture of organic acids, salts of organic acids, amides of organic acids. Additionally, during processing of ammonium salts of lactic acid, there is a tendency for lactamide to form via the following reactionLactic acid+ammonia→lactamide+waterSuch organic acid amides can be formed in fermentation broths.
Extraction can be used to aid in the recovery of organic acids from fermentation broths or other streams comprising organic acids and their salts (e.g., hydrolyzed polylactide, among others). Certain extractants or extraction enhancing agents can react with the hydroxy organic acid present in a feed stream to produce an ester of the hydroxy organic acid. Methods for producing lactide from sources comprising an ester of the hydroxy organic acid, hydroxy organic acid amides or ammonium salts of hydroxy organic acids are desirable.
As noted above, a typical process for the production of lactide involves the dehydration of a lactic acid stream to form a mixture of oligomers of lactic acid. This lactic acid stream can be the product of a fermentation or it can be derived from a chemical synthesis, and it is preferred that the lactic acid stream is at least partially purified before the dehydration step. After the dehydration step, the mixture of oligomers of lactic acid molecules is mixed with a depolymerization catalyst. The mixture is heated so that the lactic acid oligomers undergo thermal decomposition and cyclization (through a condensation reaction), thus producing lactide. This lactide is simultaneously or sequentially separated, often as a vapor stream. In such processes, the vapor can further comprise such impurities as monomer, dimer, and low molecular weight lactic acid oligomers, and water. When the lactide is produced from lactic acid that is the product of fermentation, the vapor can further comprise impurities derived from the fermentation, like sugars, amino acids, fatty acids, and organic acids other than lactic acid. The vapor comprising lactide can be optionally partially or fully condensed and then distilled or otherwise purified such as by fractional crystallization to form a “purified” lactide. Glycolide can be formed using similar methods.
In methods known in the art for the production of L-lactide from L-lactic acid, species such as L-lactide, meso-lactide, water, lactic acid, lactic acid linear dimer, lactic acid oligomers, lactic acid degradation products, other impurities, and traces of D-lactide can be present in varying concentrations. Thus, methods for producing relatively pure lactide, particularly optically pure lactide, are desirable. While glycolide and certain other cyclic esters of hydroxy organic acids do not have optical isomers, it is also desirable to have alternative methods for their production and purification. Thus, there is a long standing need for improved processes for the production and recovery of relatively pure lactide and glycolide and related cyclic esters. Particularly, processes that can produce cyclic esters of certain hydroxy organic acids, such as lactic acid and glycolic acid, in good purity directly from wet crude fermentation broth are desirable.